In machine technology and especially in the construction of coordinate measuring apparatus, it is often necessary to protect longitudinally extending, movable components which project into the work or measuring area against unintended collisions. Such machine components can be, for example, the measuring arm of a coordinate measuring apparatus with the measuring arm carrying the measuring tool in the form of a measuring head. Injury to operating personnel by the movable machine components is also to be prevented.
For this purpose, various protective devices are already known. For example, U.S. Pat. No. 4,713,892 discloses a protective device wherein an ultrasonic envelope is placed about the machine component to be protected or provides for a plurality of helical springs arranged along the length of the component which are at another electrical potential compared to the structure of the machine and bring the drive of the machine to standstill when there is a contact with the machine component.
Protective devices in the form of several light barriers which extend along the length of the machine component are disclosed in European patent publication 0,116,807 and German published patent application 3,514,444. A protective envelope is provided and formed by a plurality of light barriers arranged one next to the other. When this protective envelope is penetrated, the electronic circuit to which the light barriers are connected supplies a signal to the emergency-stop control of the particular machine.
These known protective arrangements do operate very reliably; however, a plurality of light barriers arranged close one next to the other is required to provide the reliability of operation which is adequate. This configuration is needed since no gaps can occur in the protective envelope defined by the light barriers through which, for example, projections on the object to be measured can come into contact with the measuring arm and damage the latter.
The protective arrangements described which are configured on the basis of light barriers are therefore relatively complex. Furthermore, the outlying components of the light barriers can easily become dirtied. These outlying components can, for example, include the surfaces of the light sources and the receivers as well as prisms used as may be required for beam deflection. This leads to the condition that the protective device responds even though no collision incident is present.